Continued expansion of channel capacity increases frequency bandwidth and thus necessitates improvement in shielding efficiency. The recent development of 48 channel PCM carrier system (Tl-C) and consequently increase in the equivalent frequency from 772 kHz to 1.576 MHz put additional demand on shielding efficiency of the internal shield. The near-end crosstalk requirements were tentatively established at 80dB worst case power sun, and our design per U.S. Pat. No. 3,803,340 was found to be marginal in performance, especiall in small pair count cables.
The improved shields require the use of more metal for shielding and necessitate changes in the method by which the cables are manufactured, but by novel configuarations of the shields, some of the cost of added material can be offset by more efficient use of the shield areas and the mechanical strength of the cables can be increased to withstand better the rigors of installation.
Internally shielded cables should be competitive costwise with the two separate cable approach; that is, each standard cable used for a different direction of transmission. The preferred embodiments of this invention are more economical than the use of different cables for each direction of transmission of signals. Shielded telephone cables are normally buried, and consequently their overall shields are used for lightning protection.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.